1. Field of the Invention
The present invention relates to an illumination optical unit and an image display system having the same, and more particularly, to an illumination optical unit employing a dichroic mirror wheel and an image display system including the illumination optical unit.
2. Description of the Related Art
FIG. 1 is a diagram showing an example of a conventional single panel color image display system using a color wheel. Referring to FIG. 1, the conventional single panel color image display system comprises a light source 100; a focusing lens 102, which focuses light incident from the light source 100 on a color wheel 105, which sequentially sorts out colors from the focused light in response to different color image signals of the color display system; fly eye lenses 107 and 110, which uniformize the distribution of intensity of each color light beam sorted out by the color wheel 105; an image display device 112, which performs optical modulation on each color light beam in response to an applied image signal; and a projection lens 115, which projects the modulated light beams onto a screen 118.
In the conventional single panel color image display system, the color wheel 105 transmits only a color light beam corresponding to each color filter in order of red, green, and blue with respect to light incident from the light source, i.e., a lamp 100, while the color wheel is rotating. Consequently, ⅔ of the total quantity of light is lost, decreasing light efficiency. 
FIG. 2 is a diagram showing an example of a conventional single panel color image display system using dichroic mirrors. FIG. 3 is a diagram showing optical paths in a micro lens array and a liquid crystal display (LCD) device, which are shown in FIG. 2. Referring to FIGS. 2 and 3, the conventional single panel color image display system using dichroic mirrors comprises an arc lamp, three dichroic mirrors 204R, 204G, and 204B, slanted with respect to one another, a polarization converter 205, a micro lens array 206, a field lens 208, a projection lens 209, and an image display device 211, comprising a micro lens array 206 and a liquid crystal layer 207.
The arc lamp comprises a lamp 202 which emits white light; an arc mirror 201 installed to surround one side of the lamp 202; and a condenser lens 203, which collects diverging light directly emitted from the lamp 202 and diverging light reflected from the arc mirror 201 and converts the collected diverging light into parallel light.
The white light emitted from the arc lamp is divided into a red (R) light beam, a green (G) light beam, and a blue (B) light beam by the three dichroic mirrors 204R, 204G, and 204B, respectively. The dichroic mirror 204R reflects the R light beam of the white light incident from the arc lamp and transmits the remaining color light beams. The dichroic mirror 204G reflects the G light beam of the color light beams transmitted by the dichroic mirror 204R and transmits the remaining color light beam, i.e., the B light beam. The dichroic mirror 204B reflects the B light beam.
The dichroic mirrors 204R, 204G, and 204B are disposed such that dichroic mirror 204G is slanted with respect to the dichroic mirror 204R at an angle of −θ and to the dichroic mirror 204B at an angle of +θ. Here, “+” indicates a counterclockwise direction, and “−” indicates a clockwise direction. Accordingly, the chief ray of the R light beam is incident on the micro lens array 206 at an angle of −θ with the chief ray of the G light beam, and the chief ray of the B light beam is incident on the micro lens array 206 at an angle of +θ with the chief ray of the G light beam.
In the micro lens array 206, a micro lens element 206a, comprising a plurality of cylindrical lenses, and a micro lens element 206b, comprising a plurality of cylindrical lenses, are arranged in a horizontal direction. The micro lens array 206 condenses the R light beam, the G light beam, and the B light beam, which are incident at different angles, on signal electrodes 207R, 207G, and 207B, respectively, of an image display device 211 in a striped pattern.
The image display device 211 has a structure in which the liquid crystal layer 207 is sandwiched between two transparent glass substrates 212 and 213. A transparent conductive film 214, the liquid crystal layer 207, and the signal electrodes 207R, 207G, and 207B, form a matrix structure.
In the conventional single panel color image display system having the above-described structure, the three dichroic mirrors 204R, 204G, and 204B divide white light incident from the arc lamp into an R light beam, a G light beam, and a B light beam, which are condensed on the signal electrodes 207R, 207G, and 207B, respectively, of the image display device 211. Taking into account a difference in the incident angle among the chief rays of the respective R, G, and B light beams, the signal electrodes 207R, 207G, and 207B are arranged at predetermined intervals in an horizontal direction. The signal electrodes 207R, 207G, and 207B are subpixels, which form a single image pixel.
Each unit micro lens of the micro lens elements 206a and 206b corresponds to three subpixels, which correspond to R, G, and B, respectively. The three subpixels are projected on a screen 210 through the field lens 208 and the projection lens 209 so that the three subpixels form a single image pixel. Accordingly, a viewer views an entire color image comprising these image pixels.
However, since three subpixels form a single image pixel in the above-described conventional single panel color image display system, the resolution of an LCD device decreases to ⅓. Accordingly, in order to realize the same resolution using, for example, projection type single panel image display systems using a color wheel, as disclosed in U.S. Pat. Nos. 5,633,755 and 5,159,485, the physical resolution of an LCD device must be increased three times. When the physical resolution of the LCD device is increased three times, aperture efficiency decreases, so optical efficiency decreases. In addition, yield in mass production decreases, thereby increasing manufacturing cost. Moreover, the size of the LCD device becomes too large because the physical resolution of the LCD device must be increased three times. When the size of the LCD device becomes large, the condenser lens, the field lens, or the projection lens must be large, thereby increasing manufacturing cost.